IN-VIVO, EX-VIVO, AND IN-VITRO EFFECTS OF L-NAME AND L-ARGININE ON THE METABOLISM OF THEOPHYLLINE IN THE RABBIT

It has been shown that selected isoforms of cytochrome P450 (P450) can generate nitric oxide from c-arginine analogs; however, the effect of L-arginine analogs on the catalytic activity of P450 remains unknown. To assess the effect of N-nitro-L-arginine methyl ester 4-NAME; 25 mg /kg) and L-arginine (150 mg/kg) on the activity of P450, these compoun ds were administered intravenously every 8 hr for 2 days to groups of six New Zealand rabbits. Thereafter, the biotransformation of theophyl line was documented in vivo (2.5 mg/kg iv) and ex vivo in hepatocytes of control and treated animals. In vivo, compared with control rabbits , both L-NAME and L-arginine increased theophylline plasma concentrati ons secondary to a reduction in theophylline systemic clearance by 46% and 42% (p < 0.05), respectively. Ex vivo, the effect of L-arginine a nalogs on P450 activity was documented by measuring the production of 3-methylxanthine (3MX), 1-methyluric acid (1MU), and 1,3-dimethyluric acid (1,3DMU) after incubation of theophylline (176 mu M) with hepatoc ytes for 4 hr. L-NAME reduced the formation of 3MX, 1MU, and 1,3DMU by 42%, 45%, and 32% (p < 0.05), respectively. However, L-arginine reduc ed only the formation of 3MX by 34% (p < 0.05). In the in vitro studie s, incubation of L-NAME or L-arginine with hepatocytes did not modify the bio-transformation of theophylline. It is concluded that L-NAME an d L-arginine inhibit the activity of several apoenzymes of P450, the p robable mechanism being a catalysis-dependent inhibition.